All diamond tools should be used with water when you are grinding glass. This isn't necessarily to save the diamonds or prolong the life of the tool, but to prolong your life.
When grinding glass, the outcome is ground silica dust. If inhaled it can lead to a disease called silicosis (a very unpleasant way to shake this mortal coil).
Using water with your tools will help keep glass dust to a minimum and keep your tools, your glass, and your lungs living longer.